This invention relates to the measurement of heart and lung volume, using the technique of measuring thoracic generated field potentials. A number of techniques have been devised in recent years for the measurement of heart and lung volume of patients by the application to the thorax of the patient of an oscillating electrical current, and the measurement of the resulting voltages produced in the thorax across the organs of interest.
Examples of previous proposals include those by D.W. Hill and B.W. Watson (IEE. Medical Electronics monographs (1-6) published by Peter Peregrinus Limited, 1971), W.G. Kubicek, R.P. Patterson, and D.A. Witso (Annals of the New York Academy of Sciences, vol 170, article 2, pages 724-732, 1970), and B. Tedner, D. Linnarsson, and T. Ribbe (World Congress on Medical Physics and Biomedical Engineering 1982, Hamburg).
In these previously proposed methods of measurement, in general, potential differences are measured using electrodes applied to the body externally, either as point electrodes, or on either side of the chest, or as an array of strip electrodes applied longitudinally or circumferentially around the neck and lower thorax.
In a paper in Anesthesia (1983 Volume 38, pages 892-897), A.B. Baker and C. McLeod refer to the use of an oesophageal electrical probe for various measurements within the chest region, for example ECG, pacing, temperature, sound measurement, and pressure measurement.
In the paper by Baker and McLeod, reference is also made to the measurement of transthoracic impedence, using an external circumferential electrode system, with an internal oesophageal electrode system, or using a system with all internal electrodes.
Inherent in impedence measurement techniques is an assumption that all of the applied current flows between the voltage measuring electrodes. Given this assumption Ohm's Law can be applied to the known constant current and the measured potential difference to calculate the impedence of the tissues lying between the measurement electrodes. If an electric current is applied to the thorax and measurements of the potential difference between points within the thorax are made, then no reliable assumption can be made on the value of current flowing between these measuring electrodes. The generated field potential can be measured but no calculation of impedence of the tissues between the electrodes is possible. Changes in the generated field potential will arise from changes in the distribution of the field within individual organs within the thorax. Thus by positioning a pair of electrodes on either side of an individual organ, e.g. the heart, changes in the generated field potential will predominately reflect changes in the impedence of that organ; the changes in the impedence of the organ will in turn arise from changes in its volume and content.
We have now found that, by the careful placement of an external electrode on the chest, coupled with the use of an electrode in the oesophagus such as that suggested by Baker and McLeod, it is possible to localize accurately the volume responsible for voltage changes, and thus to measure cardiac volume, and by measuring the change of cardiac volume with time, cardiac stroke volume.
Accordingly, in a first aspect of the invention, there is provided a method of measuring cardiac activity, which method comprises
generating an oscillating electrical signal, PA1 applying the oscillating electrical signal to the thorax of a patient, PA1 providing a second electrode on the chest of the patient in a position such that measurement of voltage between the said first and second electrodes provides PA1 a measure of cardiac volume, and PA1 measuring the voltage induced between the first and second electrodes and producing therefrom an output signal indicative of cardiac activity in the patient. PA1 means for generating an oscillating signal, PA1 means for applying the oscillating electrical signal to the thorax of a patient, PA1 an oesophageal probe including a first electrode, PA1 a second electrode adapted to be positioned on the chest of the patient in the region of the apex of the heart, whereby measurement of voltage between the said first and second electrodes provides a measure of heart volume, PA1 a third electrode adapted to be positioned on the chest of the patient at a position on the right side of the chest generally corresponding with that of the second electrode of the left side of the chest, whereby measurement of voltage between the said first and third electrodes provides a measure of volume of the right lung of the patient, and PA1 means for measuring the voltage induced between the first and second, and the first and third electrodes respectively, and for thereby providing first and second output signals respectively, indicative of heart volume and lung volume respectively.
We have also discovered that by placement of a further electrode on the chest, it is possible to form separate assessments using a single piece of apparatus of heart stroke volume, and also lung tidal volume. This is achieved by so constructing the apparatus as to provide two measurements of generated potentials in selected locations, rather than one. In a further development, we have discovered that, by applying to a voltage measurement indicative of heart stroke volume, a correcting signal derived from the lung volume signal, it is possible to exclude from the heart stroke volume signal a substantial amount of "noise", due to fluctuating changes in trans-pulmonary voltage. Similarly, by applying a correcting signal derived from this heart volume signal to the lung volume signal, it is possible to improve the signal to noise ratio of the lung volume signal.
Accordingly, in a first aspect of the invention, there is provided apparatus for measuring heart volume and/or lung volume, which apparatus comprises
The apparatus preferably includes means for varying either one of the output signals in dependence upon the other of the said output signals, for example by adding to the respective output signal a correcting signal, which may be simply the other signal, with an appropriate amplitude modification, or phase shift. Means may be provided for varying automatically the proportion of, for example, the lung signal which is used as a correcting signal to the heart signal.
In general, the oscillating electrical signal will be a constant current signal, and this is preferably applied to the thorax of the patient by means of a pair of electrodes applied externally of the thorax. Each electrode of this electrode pair may preferably be provided in an electrode pad assembly, which also includes an appropriate one of the heart or lung sensing electrodes.
The two electrode pairs may be combined into a single four electrode assembly which can then simply be positioned across the front of the chest as a single assembly.